Separation of C2F6 from CF4 by adsorption on activated carbon

ABSTRACT

This invention relates to an improvement in a process for removing C 2 F 6  as an impurity from a CF 4  containing gas, preferably CF 4  produced by the reaction of F 2  with carbon. The improvement in the process comprises the steps: 
     contacting the CF 4  containing gas, containing C 2 F 6  impurity, with an activated carbon having a CCl 4  activity from 43 to 55 in an adsorption bed to effect adsorption of the C 2 F 6  impurity; and, 
     recovering purified CF 4  product in the effluent from the adsorbent bed.

BACKGROUND OF THE INVENTION

Historically, high purity CF₄, which is used to etch silica in themanufacture of integrated circuits, has been derived by the directfluorination of carbon. Purification of the CF₄ gas formed, whichincludes the removal of impurities such as C₂F₆, has been effected bytemperature swing adsorption using a zeolite bed, such as, NaX (13X).CF₄ is co-adsorbed with the C₂F₆ impurity on the 13X zeolite. When thebed is saturated with C₂F₆, a N₂ purge is passed through the spentadsorbent bed in order to recover some of the coadsorbed CF₄ from thebed. The CF₄, which is less strongly adsorbed than the C₂F₆, desorbsfirst into the N₂ purge gas and then the C₂F₆ is desorbed. The N₂ purgegas containing desorbed CF₄, and small amounts of desorbed C₂F₆, ispassed through a second bed of 13X zeolite until the C₂F₆ concentrationin the effluent rises to an unacceptable level in the CF₄/N₂ mixture. Bythis process about half of the co-adsorbed CF₄ can be recovered beforethe C₂F₆ concentration in the effluent becomes too high.

Representative patents relating to the separation of carbon fluoridegases are as follows:

U.S. Pat. No. 6,187,077 discloses a process for separating at least oneof CF₄ and C₂F₆ from a gas containing at least one of NF₃, CHF₃ and N₂and SF₆. The process steps include 1) passing a feed stream containingvarious impurities through a glassy membrane to produce a retentatestream rich in SF₆ and at least one of CF₄ and C₂F₆, and, then 2)contacting the retentate stream with an adsorbent effective to adsorbSF₆ and produce a product stream rich in at least one of CF₄ and C₂F₆.Representative adsorbents include zeolites, preferably X types,activated carbons, e.g., BPL, (data sheet indicates a CCl₄ activity of60-65), PCB (data sheet indicates a CCl₄ activity of 60), BAC, F-300,F-400, BPL, RB2 (data sheet indicates an activity of 65) with PCB beingthe preferred activated carbon. Polymeric adsorbent resins, and carbonmolecular sieves are also disclosed.

U.S. Pat. No. 5,523,499 discloses a process for the purification of C₂F₆contaminated with CClF₃ and CHF₃ impurities by adsorption. The C₂F₆ gascontaminated with impurities is contacted with a sorbent which includeszeolite molecular sieves and activated carbons. Preferred activatedcarbons, such as BPL from the Calgon Corporation and Type UU fromBarneby and Sutcliffe Corp having a particle size of from 4 to 325 mesh.

Japanese Patent Application No. 54-62867 (disclosure 55-154925)discloses a purification process for CF₄ containing CF₃Cl as animpurity. The process comprises the steps of irradiating the gas streamwith a laser and absorbing photons in the fluorine compounds therebyconverting the CF₃Cl to C₂F₆ and Cl₂. The C₂F₆ is then removed viadistillation or adsorption.

There is a need in the industry for adsorbents that would allow for along onstream time for a given column size and a need for adsorbentswhich have a higher selectivity for carbon fluoride impurities otherthan the CF₄ product. Such improved adsorbents would enhance therecovery of CF₄.

BRIEF SUMMARY OF THE INVENTION

This invention relates to an improvement in a process for removing C₂F₆as an impurity from a CF₄ containing gas, preferably CF₄ produced by thereaction of F₂ with carbon. The improvement in the process comprises thesteps:

contacting said CF₄ containing gas with an activated carbon having aCCl₄ activity from 43 to 55 in an adsorption bed to effect selectiveadsorption of said C₂F₆ impurity; and,

recovering a purified CF₄ product in the effluent from said adsorbentbed.

Significant advantages of this process include:

an ability to remove impurities from a CF₄ containing gas streamcontaminated with fluorocarbon impurities;

an ability to selectively adsorb contaminant C₂F₆ without effectingsubstantial losses to irreversible adsorption of CF₄;

an ability to provide for long onstream times in the adsorption process;and,

an ability to achieve effective removal of the fluorocarbon impuritiesfrom the CF₄ product.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph of C₂F₆ adsorption on activated carbon versus the CCl₄activity of such activated carbon adsorbents.

DETAILED DESCRIPTION OF THE INVENTION

Carbon tetrafluoride is produced by direct fluorination of carbon. Inthat process a reaction product is generated which contains manyimpurities which must be removed. Various impurities in the reactionproduct include unreacted fluorine, HF, SF₆, and carbon fluorides, suchas, CHF₃ and C₂F₆. The impurity C₂F₆ is generated from the incompletereaction of carbon with F₂. Typical CF₄ feed gas concentrations havefrom 500 to 5000 ppm C₂F₆ in CF₄ and the C₂F₆ must be reduced to lessthan 0.5 ppm for commercial high purity CF₄.

Removal of contaminant C₂F₆ from a CF₄ gas stream to produce commercialhigh purity CF₄ is achieved by contacting the gas stream with anactivated carbon having a carbon tetrachloride activity coefficient offrom 43 to 55. If the binding power of the activated carbon is below aCCl₄ activity of 43, then some of the C₂F₆ may pass through andcontaminate the CF₄ product. If higher than 55, the binding power of theactivated carbon is too high and CF₄ product may be irreversiblyretained within the adsorbent bed of activated carbon. This level ofactivity allows for C₂F₆ adsorption capacities of 0.15 millimolesC₂F₆/gram activated carbon (mmole C₂F₆/gram), preferably at least 0.02and most preferably at least 0.025 mmC₂F₆/gram activated carbon at atemperature from 25 to 30° C.

The carbon tetrachloride activity of an adsorbent is a standardmeasurement (ASTM test method D3467-99) that measures the gravimetricsaturation capacity of the adsorbent. Carbon tetrachloride (CCl₄)activity is defined herein as the ratio (in percent) of the weight ofCCl₄ adsorbed by an activated carbon sample to the weight of the sample,when the carbon is saturated with CCl₄ under conditions listed in thisASTM test method and is incorporated by reference. Recently, the use ofCCl₄ activity has been replaced by characterization of the adsorption ofbutane on the adsorbent. Butane activity is equivalent to CCl₄ activityand is equated to CCl₄ activity per the equation:

Butane activity=CCl₄ activity/2.52

Thus, a corresponding CCl₄ activity can be obtained by multiplying thebutane activity of the adsorbent by 2.52.

The activated carbon suited for use in removing the impurity C₂F₆typically has a particle size range of 0.5 to 3 mm in diameter.

Inlet feed temperatures to the adsorption bed range from 0 to 100° C.and inlet feed pressures to the adsorption bed range from 1 to 20 atmabsolute. Preferred operation temperatures range from 20 to 50° C. andpressures from 2 to 10 atm.

Regeneration of the spent carbon bed for reuse can be effected inaccordance with generally accepted procedures. Desorption can beachieved at pressures from 0.1 to 2 atm absolute and temperatures from50 to 300° C. Accordingly, the process lends itself to both a thermalswing and pressure swing adsorption/desorption process. Regeneration ofthe bed using thermal swing adsorption allows for the use of nonreactivegases as a purge. These gases include N₂, Ar, He, H₂ and mixturesthereof.

Although adsorbents having a carbon tetrachloride adsorption capacity ofbetween 43 and 55 are perferentially selective for C₂F₆ than CF₄, someloses of CF₄ occur in adsorption. Modest recovery of CF₄ adsorbed by theactivated carbon in the adsorption bed can be achieved by passing apurge gas through the spent carbon bed. CF₄ is desorbed into the purgegas in preference to C₂F₆. To remove residual C₂F₆ from the purge gas,the purge gas from the spent gas is passed through a fresh bed ofactivated carbon wherein the residual C₂F₆ is adsorbed. Enhancedrecovery of CF₄ is achieved. Multiple beds with pressure equalizationsteps can be used to improve recovery.

The following examples are provided to illustrate preferred embodimentsof the invention and are not intended to restrict the scope thereof.

EXAMPLE 1 Selective Adsorption of CF₄ on Activated Carbon

To determine the effectiveness of various activated carbons for removalof C₂F₆ from CF₄, 198 g of Pacific Activated Carbon (CCl₄ activity is45) were packed into a 1″ diameter column with a length of 3 feet orapproximately 1 meter. Feed CF₄ gases were passed through the column andthe effluent from the column was analyzed for percent concentration ofCF₄ and the concentration of C₂F₆ in ppm by gas chromatography (GC).

Initially, the column filled with Pacific Activated Carbon (CCl₄activity 45) was saturated with CF₄ and allowed to come to a constanttemperature. Then, a gas mixture containing 1000 ppm of C₂F₆ and 300 ppmSF₆ in CF₄ was flowed through the column operating at atmosphericpressure and a temperature of 25 to 30° C. through at 100 sccm. Theeffluent from the column was analyzed every 3 minutes until there was abreakthrough. The breakthrough time was defined as the time 3 ppm ofC₂F₆ was detected in the effluent by GC. After 20.0 hours of operation,C₂F₆ breakthrough was observed. This corresponds to a C₂F₆ capacity ofthe Pacific Activated Carbon of 0.026 mmole C₂F₆/g at breakthrough.

EXAMPLE 2 Selective Desorption of CF₄ from Activated Carbon

Directly after breakthrough of the C₂F₆ in Example 1, the feed gas wasswitched to a flow of 100 sccm N₂ to effect desorption of the bed. Thecolumn was operated at ambient temperature during desorption. Duringinitial desorption of the CF₄ from the activated carbon bed, theimpurity level in the purge gas remained constant at 40 ppm of C₂F₆.After purging for 106 minutes the CF₄ concentration in the N₂ droppedbelow 10% and desorption was stopped.

The desorbed CF₄ in the N₂ could be recovered by conventional means,e.g., by passing the CF₄/N₂ mixture through a second activated carboncolumn to remove the C₂F₆ and cryogenically separating from the purgeN₂.

EXAMPLE 3 Selective Adsorption of CF₄ with 13X Zeolite

The procedure of Example 1 was repeated except 278 g of 13X (CCl₄activity 38) was used as the adsorbent in place of the Pacific ActivatedCarbon. A CF₄ feed gas contaminated with both SF₆ and C₂F₆ was used andbreakthroughs were observed after 13.0 hours. This corresponds to a C₂F₆capacity of 0.012 mmole/g.

EXAMPLE 4 Recovery of CF₄ from 13X Zeolite

A CF₄ recovery experiment similar to Example 2 was performed on thecolumn employed in Example 3. Nitrogen was passed through the bed andafter 60 minutes the CF₄ concentration in the N₂ dropped below 10%.Desorption was stopped at that point. The C₂F₆ concentration in thenitrogen was observed at 60 ppm and steadily rose to 285 ppm at the endof the desorption step for the recovery of CF₄.

EXAMPLE 5 Adsorption of CF₄ on Activated Carbon

The procedure of Example 1 was repeated except for the use of adifferent activated carbon. A breakthrough experiment was conductedunder the same conditions with Barneby-Sutcliffe type 205A activatedcarbon adsorbent having a CCl₄ activity of 53. The C₂F₆ capacitydetermined from the breakthrough curve was 0.016 mmole/g. The activedensity of the type 205A carbon is essentially the same as that for thePacific activated carbon (0.5 g/cc).

EXAMPLE 6 Plot of C₂F₆ Capacity Verses CCl₄ Activity

A plot of C₂H₆ adsorption in mm/gram activated carbon versus CCl₄activity of the activated carbon was prepared based upon the results ofthe above examples is set forth in FIG. 1.

The results in the plot show a surprising result. The C₂F₆ capacity ofthe absorbents goes through a desirable range where the activity is atleast 43 to 55 with a preferred range of from 44 to 50 with a maximumCCl₄ activity at about 45. This is an unexpected result since one wouldexpect the higher the saturation capacity of the adsorbent, the higherthe C₂F₆ capacity. From the data, activated carbons having a highcapacity for C₂F₆ do not have the same high capacity for CF₄. Activitylevels of 0.015 and preferably from about 0.02 to about 0.03 mmoleC₂F₆/gram activated carbon allow for CF₄ to pass through in the effluentand allow for significant levels of recovery by purging from theactivated carbon without substantial contamination from the C₂F₆.

Summarizing from the examples above, it is clear that the PacificActivated Carbon having a CCl₄ activity of 45 outperformed both thezeolite 13X which had a CCl₄ activity of 38 and the Barneby-Sutcliffeactivated carbon which had a CCl₄ activity of 53. However, theBarneby-Sutcliffe activated carbon has sufficient activity for removingC₂F₆ for some applications. Comparing Examples 1 and 3, the PacificActivated Carbon is shown to have more than twice the capacity for C₂F₆compared to 13X on a mmole/g basis. Also, due to the differences inpacking densities, activated carbon allows for on-stream times >50%longer for removing C₂F₆ from CF₄ than 13X. Moreover, these activatedcarbons adsorb the residual impurity, SF₆, much more strongly than does13X.

Another feature of the use of activated carbon having an activitybetween 43 and 55 is noted in the comparison of Examples 2 and 4. Theseexamples show that recovery of CF₄ at ambient temperature using a N₂purge can be achieved. There is C₂F₆ and essentially no SF₆ desorbed inthe purge gas when an activated carbon having a CCl₄ activity between 43and 55 is used versus 13X.

The examples also show that the presence of low levels of SF₆ in thefeed does not adversely affect the measured capacity of C₂F₆ in Examples1 or 5.

The present invention has been described with several preferredembodiments, but the full scope of the invention should be ascertainedfrom the claims which follow.

What is claimed is:
 1. In a process for removing C₂F₆ as an impurityfrom a CF₄ containing gas, wherein the CF₄ containing gas is contactedwith an adsorbent in an adsorbent bed for selectively adsorbing C₂F₆,and the CF₄ is recovered in the effluent from the adsorbent bed, theimprovement which comprises the steps: contacting said CF₄ containinggas, containing C₂F₆ impurity, with an adsorbent comprised of anactivated carbon having a CCl₄ activity from 43 to 55 in an adsorptionbed to effect adsorption of said C₂F₆ impurity; and, recovering apurified CF₄ product in the effluent from said adsorbent bed.
 2. Theprocess of claim 1 wherein the CF₄ containing gas has from about 500 to5000 ppm C₂F₆ as an impurity.
 3. The process of claim 2 wherein theactivated carbon has a particle size of from 0.5 to 3 mm diameter. 4.The process of claim 3 wherein the adsorbent bed is operated at atemperature from 0 to 100° C. and inlet feed pressures to the adsorptionbed range from 1 to 20 atm absolute.
 5. The process of claim 2 whereinenhanced recovery of CF₄ is achieved by passing a purge gas through thebed to selectively desorb CF₄ from the bed prior to desorption of C₂F₆.6. The process of claim 5 wherein the purge gas is nitrogen.
 7. Theprocess of claim 6 wherein CF₄ retained in the adsorption bed isdesorbed at a pressure from 0.1 to 2 atmospheres and a temperature from50 to 300° C.
 8. In a process for removing C₂F₆ as an impurity from aCF₄ containing gas, wherein the CF₄ containing gas is contacted with anadsorbent in an adsorbent bed for selectively adsorbing C₂F₆, and theCF₄ is recovered in the effluent from the adsorbent bed, the improvementwhich comprises the steps: contacting said CF₄ containing gas,containing C₂F₆ impurity, with an activated carbon having a capacity forC₂F₆ of at least 0.015 millimoles C₂F₆ per gram of activated carbon inan adsorption bed at a temperature from 25 to 30° C. to effectadsorption of said C₂F₆ impurity; and, recovering a purified CF₄ productin the effluent from said adsorbent bed.
 9. The process of claim 8wherein the CF₄ containing gas has from about 500 to 5000 ppm C₂F₆ as animpurity.
 10. The process of claim 8 wherein the activated carbon has acapacity for C₂F₆ of at least 0.02 to 0.03 millimoles C₂F₆ per gram ofactivated carbon.